my dear mr. darwin!

your two so very kind letters have given me the greatest pleasure, and i thank you very much for having taken such a trouble in answering my question. i am glad to see, that i did not misunderstand you in supposing, that mr. wagner had misrepresented your views with respect to his theory.¹

the question arises now, what to do? i shall have to refer to wagners views in my work on the influence of external conditions of existence on animals; and i am bound to state, that to my understanding his theory appears to be only an integral part of your more general theory; yet he asserts in the most positive way, that you yourself appear to incline most decidedly in favour of his views.² not having any right, to use your letters against wagner, i fear, all german readers of my work will say, and mr wagner certainly will do so, that i had no ground for interpreting your unknown words in your letters to wagner in a way different from that, to which wagner thinks himself justified. now if you will entitle me, to make use of your letters in the discussion of these matters, i certainly shall use them in the most discretionary way; if you do not wish to be drawn into the discussion directly, please tell me so candidly and i assure you, that not a single word of what you wrote me, will appear in print.

i thank you also for the many valuable remarks, you gave me in your letters on the subject of the influence of isolation and external conditions on animals and i am glad to see, that you do not regard this latter as altogether valueless.³ for i must confess, that i attach greater importance to this agency, than you seem to be inclined to do. supposing then, that selection and external conditions have acted together, it seems to be the principal question, to ask how much of the ultimate effect may have been due to the one or the other. and this question can be answered, as far as i see, only by keeping both causes apart in experimental researches. to direct the attention of the speculating zoologists to such experimental work and to induce them, to abstain for some time at least from speculating in such a random way as has been done f. i. by weissmann or by jaeger, to try to do this, is the main object of the work, i am writing now.⁴ during this time, i fear, i shall not have the time for treating the isolation question as thoroughly, as i would like to do. but after having finished the book in question, i hope to get more leisure, than i have just now and in that case, it is my firm intention to discuss the wagnerian hypothesis to some extent and on the same ground, restricted as it is, which has been taken by wagner for exemplifying.

remember me please to your family and believe me to be | yours devotedly | C. Semper⁵